1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, and more particularly to an ignition coil having a permanent magnet disposed in a magnetic circuit.
2. Description of the Related Art
In an ignition system for an internal combustion engine, when a primary current in an ignition coil is intermittently interrupted, a high voltage is obtained from a secondary winding in proportion to the rate of variation of the magnetic flux produced in a core and delivered to an ignition plug to ignite a mixture within a cylinder of the engine.
According to a recent internal combustion engine with its power output increased, the ignition coil requires that its output voltage and discharge energy are increased. Therefore, it is necessary that the cross sectional area of the core is increased, and/or the number of turns of the secondary winding mounted on the core is increased. By so doing, however, the size of the ignition coil will be made larger against a demand for the ignition system of its size reduced as a whole.
As well known and described in Japanese Utility Model Laid-open Publication No. 48-49425, the number of turns of the secondary winding should be increased or the magnetic flux passing through the core should be increased in order to increase the output voltage of the secondary winding. In that Publication, there has been proposed an ignition coil which includes a magnet disposed in a magnetic circuit for providing a magnetizing force in the direction opposite to the magnetization of the coil in case of closing of a switch for feeding an electric current to the coil. Also, Japanese Patent Publication No. 41-2082 discloses an ignition coil which has a permanent magnet disposed in a magnetic circuit of an iron core to provide the magnetic flux differential to, i.e., opposite to the magnetic flux created in a primary winding. Japanese Patent Laid-open Publication Nos. 59-167006 and 60-218810 disclose an ignition coil having a permanent magnet which is disposed in a gap provided in a core. In any of those described above, the core is provided with a gap at a position other than the position on which the primary and secondary windings are mounted, and the permanent magnet is disposed in the gap.
In the ignition coil having the permanent magnet disposed in the magnetic circuit as mentioned above, the magnetic flux variation produced in response to the intermittent interruption of the primary current is increased, so that the output voltage obtained from the secondary winding is increased in comparison with the conventional ignition coils. However, in such ignition coil, since a great leakage of magnetic flux is created when the electric current is fed to the primary winding, most of the increased magnetic flux is offset by the leaked magnetic flux, so that the increasing rate of the magnetic flux is low. Further, since the permanent magnet is disposed so as to provide a magnetic flux in a direction opposite to the direction of the magnetic flux which is produced when the primary winding is fed with the electric current, the permanent magnet will possibly be demagnetized to thereby reduce the ignition performance.
As for the ignition system for the internal combustion engine, it has been requested more and more to reduce its size. In Japanese Patent Laid-open Publication No. 63-109265 for example, therefore, it has been proposed that the ignition coil is connected directly to the ignition plug, and that the ignition coil is disposed within a cylinder head cover. Then, it is required that the ignition coil is made smaller in size by reducing an area of a cross section of the core perpendicular to the axis of the core. By so doing, however, ignition performances such as an output voltage or a discharge energy will be reduced. Thus, it is necessary for the current ignition coil to fulfill such requirement opposing to each other as the increase of ignition performance on one hand and the reduction in size on the other hand.